Method and apparatus for operating a power save mode in a wireless LAN system

ABSTRACT

A communication method that includes transmitting a power save multi-poll (PSMP) frame containing a power save mode (PSM) of a non-access point (AP) station (STA) to a first station and to a second station which are paired with an AP on the basis of multiple input multiple output (MIMO) technology. Information on the PSM indicates the point in time of starting a downlink transmission period, the duration of the downlink transmission period, and transmission object station identification information. The method further involves transmitting a data frame to the first station and to the second station on the basis of the MIMO technology during the downlink transmission period. The transmission object station identification information indicates a group identifier for indicating an object station group including the first station and the second station.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser.No.14/883,887, filed on Oct. 15, 2015, now U.S. Pat. No. 9,673,879, whichis a continuation of U.S. patent application Ser. No. 13/700,123, filedon Nov. 26, 2012, now U.S. Pat. No. 9,191,893, which is the NationalStage filing under 35 U.S.C. 371 of International Application No.PCT/KR2011/003842, filed on May 25, 2011, which claims the benefit ofU.S. Provisional Application No. 61/348,253, filed on May 26, 2010, thecontents of which are all hereby incorporated by reference herein intheir entirety.

TECHNICAL FIELD

The present invention relates to a wireless local area network (WLAN)system, and more particularly, to a method for a power save modeoperation performed by an access point in the WLAN system, and anapparatus supporting the method.

BACKGROUND ART

With the advancement of information communication technologies, variouswireless communication technologies have recently been developed. Amongthe wireless communication technologies, a wireless local area network(WLAN) is a technology whereby Internet access is possible in a wirelessfashion in homes or businesses or in a region providing a specificservice by using a portable terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

The IEEE 802.11n is a technical standard relatively recently introducedto overcome a limited data rate which has been considered as a drawbackin the WLAN. The IEEE 802.11n is devised to increase network speed andreliability and to extend an operational distance of a wireless network.More specifically, the IEEE 802.11n supports a high throughput (HT),i.e., a data processing rate of up to above 540 Mbps, and is based on amultiple input and multiple output (MIMO) technique which uses multipleantennas in both a transmitter and a receiver to minimize a transmissionerror and to optimize a data rate. The power save mode is supported forpower saving of an STA which does not require the active state toreceive the frame. An STA supporting the power save mode (PSM) can avoidunnecessary power consumption by operating in a doze mode when it is nota time duration in which the STA can access to its radio medium. Thatis, the STA operates in the awake state only for a time duration inwhich a frame can be transmitted to the STA or a time duration in whichthe STA can transmit the frame. For this, an access point (AP) composinga Basic Service Set (BSS) provides non-AP STAs associated with the APand/or which located in a BSS service area (BSA) with information abouta PSM operation. Therefore, STAs supporting the PSM can transmit andreceive frame by switching an operation mode between a sleep mode and anawake mode based on the corresponding information.

With the widespread use of the WLAN and the diversification ofapplications using the WLAN, there is a recent demand for a new WLANsystem to support a higher throughput than a data processing ratesupported by the IEEE 802.11n. A next-generation WLAN system supportinga very high throughput (VHT) is a next version of the IEEE 802.11n WLANsystem, and is one of IEEE 802.11 WLAN systems which have recently beenproposed to support a data processing rate of above 1 Gbps in a MACservice access point (SAP).

A next-generation WLAN system supporting supports a Multi User-MultipleInput Multiple Output (MU-MIMO) transmission which a plurality of STAssimultaneously access a channel for efficiently using a wirelesschannel. By using the MU-MIMO transmission, an AP can simultaneouslytransmit frames to one or more non-AP STA which is MIMO pared to the AP.

Meanwhile, although the AP provides information for operating the PSMmode to the non-AP STA in the conventional WLAN system, an aspect to beconsidered for MU-MIMO has not been adopted. Therefore, there is a needto discuss on a method for supporting a PSM mode operation of one ormore non-AP STAs in a next-generation WLAN system supporting MU-MIMOtransmission.

SUMMARY OF INVENTION Technical Problem

The present invention provides a method and apparatus for operating apower save mode in a next generation wireless local area network (WLAN)system supporting multi user-multiple input multiple output (MU-MIMO)transmission.

Technical Solution

In an aspect, a method for operating a power save mode (PSM) in awireless local area network (WLAN) system is provided. The methodincludes transmitting a power save multi-poll (PSMP) frame containingPSM information of a non-access point (AP) station (STA) to a first STAand a second STA which are multiple input multiple output (MIMO)-pairedwith an AP, the PSM information including a downlink transmission startoffset indicating a time point at which a downlink transmission periodstarts, a downlink transmission duration indicating a duration of thedownlink transmission period, and transmission destination STAidentification information, and transmitting a data frame to the firstSTA and the second STA by using MIMO transmission during the downlinktransmission period. The transmission destination STA identificationinformation indicates a group identifier (ID) for indicating adestination STA group including the first STA and the second STA whichare destinations of MIMO transmission performed by the AP.

The PSM information may further include uplink transmission start offsetinformation indicating a time point at which the first STA can accessradio media to transmit the data frame to the AP and first uplinktransmission duration information indicating a duration in which thefirst STA can access the radio media from the time point.

The method may further include transitioning the first STA and thesecond STA to an awake mode at a time point indicated by the downlinktransmission start offset information.

The method may further include operating the second STA in a sleep modeduring a period indicated by the first uplink transmission durationinformation.

The method may further include transmitting a first data frame from thefirst STA to the AP during a period indicated by the first uplinktransmission duration information.

Information for the PSM operation may further include second uplinktransmission duration information for indicating a period in which thesecond STA can access radio media to transmit the data frame to the AP.

The method may further include operating the first STA to transition toa sleep mode during a period indicated by the second uplink transmissionduration information.

The method may further include transmitting a second data frame from thesecond STA to the AP during the period indicated by the second uplinktransmission duration information.

In another aspect, a wireless apparatus includes a transceiver fortransmitting and receiving a radio signal, and a processor operativelycoupled to the transceiver. The processor is configured for transmittinga power save multi-poll (PSMP) frame containing PSM information of anon-access point (AP) station (STA) to a first STA and a second STAwhich are multiple input multiple output (MIMO)-paired with an AP, thePSM information including a downlink transmission start offsetindicating a time point at which a downlink transmission period starts,a downlink transmission duration indicating a duration of the downlinktransmission period, and transmission destination STA identificationinformation; and transmitting a data frame to the first STA and thesecond STA by using MIMO transmission during the downlink transmissionperiod. The transmission destination STA identification informationindicates a group identifier (ID) for indicating a destination STA groupincluding the first STA and the second STA which are destinations ofMIMO transmission performed by the AP.

The PSM information may further include uplink transmission start offsetinformation indicating a time point at which the first STA can accessradio media to transmit the data frame to the AP and first uplinktransmission duration information indicating a duration in which thefirst STA can access the radio media from the time point.

Advantageous Effects

In a next generation wireless local area network (WLAN) systemsupporting multi user-multiple input multiple output (MU-MIMO)transmission, information for a power save mode (PSM) operation of aplurality of stations (STA) which are MIMO-paired with an access point(AP) is transmitted by inserting the information into a power save multipoll (PSMP) frame. As a result, downlink transmission can be performedby using a MU-MIMO transmission method even if the PSM operation isperformed, an overall throughput of the WLAN system can be improved, andpower of the STA can be effectively saved.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing the configuration of a WLAN system to whichembodiments of the present invention may be applied.

FIG. 2 shows an example of a PSM operation based on a PSMP.

FIG. 3 shows another example of a PSM operation based on a PSMP.

FIG. 4 is a diagram showing an example of a PSMP STA info field format.

FIG. 5 is a diagram showing another example of a PSMP STA info fieldformat.

FIG. 6 shows an example of a PSM operation according to an embodiment ofthe present invention.

FIG. 7 is a diagram showing an example of a PSMP STA info field formataccording to an embodiment of the present invention.

FIG. 8 shows a PSMP group address ID applicable to an embodiment of thepresent invention.

FIG. 9 is a diagram showing another example of a PSMP STA info fieldaccording to another embodiment of the present invention.

FIG. 10 is a diagram showing another example of a PSMP STA info fieldaccording to another embodiment of the present invention.

FIG. 11 is a block diagram showing a wireless apparatus for implementingan embodiment of the present invention.

MODE FOR INVENTION

FIG. 1 is a diagram showing the configuration of a WLAN system to whichembodiments of the present invention may be applied.

Referring to FIG. 1, A WLAN system includes one or more Basic ServiceSet (BSSs). The BSS is a set of stations (STAs) which can communicatewith each other through successful synchronization. The BSS is not aconcept indicating a specific area.

An infrastructure BSS includes one or more non-AP STAs STA1, STA2, STA3,STA4, and STA5, an AP (Access Point) providing distribution service, anda Distribution System (DS) connecting a plurality of APs. In theinfrastructure BSS, an AP manages the non-AP STAs of the BSS.

On the other hand, an Independent BSS (IBSS) is operated in an Ad-Hocmode. The IBSS does not have a centralized management entity forperforming a management function because it does not include an AP. Thatis, in the IBSS, non-AP STAs are managed in a distributed manner. In theIBSS, all STAs may be composed of mobile STAs. All the STAs form aself-contained network because they are not allowed to access the DS.

The STA is an arbitrary functional medium including a medium accesscontrol (MAC) and wireless-medium physical layer interface conforming tothe institute of electrical and electronics engineers (IEEE) 802.11standard, and includes both an AP and a non-AP STA in a broad sense.

The non-AP STA is an STA which is not an AP. The non-AP STA may also bereferred to as other terms, such as a mobile terminal, a wirelesstransmit/receive unit (WTRU), a user equipment (UE), a mobile station(MS), a mobile subscriber unit, etc.

The AP is a functional entity for providing connection to the DS througha wireless medium for a non-AP STA associated with the AP. Althoughcommunication between non-AP STAs in an infrastructure BSS including theAP is performed via the AP in principle, direct communication is alsopossible between the non-AP STAs when a direct link is set up. The APmay also be referred to as a central controller, a base station (BS), anode-B, a base transceiver system (BTS), a site controller, etc.

A plurality of infrastructure BSSs including the BSS shown in FIG. 1 canbe interconnected by the use of the DS. An extended service set (ESS) isa plurality of BSSs connected by the use of the DS. STAs included in theESS can communicate with each another. In the same ESS, a non-AP STA canmove from one BSS to another BSS while performing seamlesscommunication.

In a WLAN system based on IEEE 802.11, a basic access mechanism of amedium access control (MAC) is a carrier sense multiple access withcollision avoidance (CSMA/CA) mechanism. The CSMA/CA mechanism is alsoreferred to as a distributed coordinate function (DCF) of the IEEE802.11 MAC, and basically employs a “listen before talk” accessmechanism. In this type of access mechanism, an STA senses a wirelesschannel or medium before starting transmission. As a result of sensing,if it is determined that the medium is in an idle status, frametransmission starts by using the medium. Otherwise, if it is sensed thatthe medium is in an occupied status, the STA does not start itstransmission but sets and waits for a delay duration for medium access.

The CSMA/CA mechanism also includes virtual carrier sensing in additionto physical carrier sensing in which the STA directly senses the medium.The virtual carrier sensing is designed to compensate for a problem thatcan occur in medium access such as a hidden node problem. For thevirtual carrier sensing, the MAC of the WLAN system uses a networkallocation vector (NAV). The NAV is a value transmitted by an STA,currently using the medium or having a right to use the medium, toanther STA to indicate a remaining time before the medium returns to anavailable state. Therefore, a value set to the NAV corresponds to aperiod reserved for the use of the medium by an STA transmitting acorresponding frame.

Meanwhile, a high throughput (HT) WLAN system conforming to the IEEE802.11n standard specifies a PSM operation based on a power save multipoll (PSMP) protocol. In the operation based on the PSMP protocol, theAP respectively allocates a downlink transmission time (DTT) and anuplink transmission time (UTT) to each of non-AP STAs associated withthe AP or non-AP STAs included in a specific group, and the non-AP STAtransmits and/or receives a frame with respect to the AP only during theDTT and/or UTT allocated to the non-AP STA.

FIG. 2 shows an example of a PSM operation based on a PSMP. In theexample of FIG. 2, an AP 10 and four non-AP STAs 21, 22, 23, and 24mutually transmit and receive frames. It is assumed that the AP 10 andthe plurality of non-AP STAs 21, 22, 23, and 24 transmit and receive theframes by accessing wireless media on the basis of CSMA/CA.

Basically, in a WLAN system supporting the PSMP, the non-AP STA operatesin an awake mode during a period in which the non-AP STA can access thewireless media, and operates in a sleep mode during a period in whichthe non-AP STA cannot access the wireless medium while other non-AP STAscan access the wireless medium.

The AP 10 transmits a PSMP frame to the plurality of non-AP STAs 21, 22,23, and 24 so as to allocate a medium access period to the plurality ofnon-AP STAs 21, 22, 23, and 24 associated with the AP (step S210). ThePSMP frame includes information indicating a period in which each non-APSTA receives the frame from the AP 10, that is, a downlink transmissiontime (DTT), and information indicating a period in which each non-AP STAtransmits the frame to the AP 10, that is, an uplink transmission time(UTT). Therefore, the plurality of non-AP STAs receive the PSMP frameand can know respective time slots in which they can transmit or receivedata.

When a PSMP-DTT is allocated to the plurality of non-AP STAs 21, 22, 23,and 24, the AP 10 can transmit a data frame to the non-AP STA during thePSMP-DTT with respect to each of the non-AP STAs 21, 22, 23, and 24. TheAP 10 can transmit the data frame to the non-AP STA1 21 during aPSMP-DTT1 (step S221). The same is also applicable hereinafter to thenon-AP STA2 22, the non-AP STA3 23, and the non-AP STA4 24 (steps S222,S223, and S224). The plurality of non-AP STAs 21, 22, 23, and 24 cantransmit the data frame to the AP 10 during a PSMP-UTT allocatedthereto. The non-AP STA1 21 can transmit the data frame to the AP 10during a PSMP-UTT1 (step S231). The same is also applicable hereinafterto the non-AP STA2 22, the non-AP STA3 23, and the non-AP STA4 24 (stepsS232, S233, and S234).

The non-AP STA1 21 can receive the data frame transmitted from the AP 10during a time slot allocated as the PSMP-DTT1 (step S221), and cantransmit the data frame to the AP 10 during a time slot allocated as thePSMP-UTT1. The PSMP-DTT2, DTT2, DTT3 and the PSMP-UTT2, UTT3, UTT4 areperiods in which other non-AP STAs receive and/or transmit data frames,and since these periods can be known to the non-AP STA1 21 whichreceives the PSMP frame, the non-AP STA1 21 can transition to a sleepmode during these periods, thereby being able to achieve power saving.The same is also applicable to the non-AP STA2 22, the non-AP STA3 23,and the non-AP STA4 24.

Meanwhile, from the perspective of the AP 10, a possibility ofincorrectly setting the PSMP DTT or of missing timing is low because anamount of data to be sent to each of the plurality of non-AP STAs 21,22, 23, and 24 can be known. However, the AP 10 cannot correctly predictdata that can be transmitted to the AP 10 from the non-AP STAs 21, 22,23, and 24. As a result, a radio resource for a pre-set PSMP-UTT may bewasted. Returning to FIG. 2, there may be a case where the AP 10 expectsto receive a data frame from the STA2 22 during a PSMP-UTT2, but thedata frame is not transmitted. Since the PSMP-UTT2 is a period which hasalready been allocated to the STA2 22 by the AP 10 to transmit the dataframe, the non-AP STA1 21, the non-AP STA3 23, and the non-AP STA4 24have already transitioned to a sleep mode in that period. Therefore,there may be a problem in that the radio resource is wasted. Thisproblem may also occur when some non-AP STAs fail to correctly receive aPSMP frame even if the PSMP frame is transmitted to non-AP STAs whichare destinations of PSMP frame transmission. To compensate for thisproblem, a PSMP recovery frame can be used.

FIG. 3 shows another example of a PSM operation based on a PSMP.

Referring to FIG. 3, an AP 10 transmits a PSMP frame to a plurality ofnon-AP STAs 21, 22, 23, and 24 (step S310), and transmits the data frameto the plurality of non-AP STAs 21, 22, 23, and 24 during respectivePSMP-DTTs (step S320).

The STA1 21 transmits the data frame to the AP 10 during an allocatedPSMP-UTT1 (step S321).

Thereafter, the STA2 22 does not transmit the data frame to the AP 10during a specific period. In this case, the AP 10 transmits a PSMPrecovery frame if no information is transmitted from the STA2 afterwaiting for PIFS in a PSMP-UTT2 (step S340). However, if the PSMP-UTT2is shorter than a PIFS+PSMP recovery frame transmission period, the PSMPrecovery frame may not be transmitted.

A frame format of the PSMP recovery frame may be the same as a format ofthe PSMP frame. However, the PSMP recovery frame includes informationindicating a modified PSMP-UTT related to a non-AP STA of which aPSMP-UTT needs to be modified and includes information indicating aPSMP-UTT related to another STA to which the PSMP-UTT is previouslyallocated. In the example of FIG. 3, the PSMP recovery frame may includeinformation indicating a modified PSMP-UTT2 for the non-AP STA2 22 andinformation indicating a PSMP-UTT3 for the non-AP STA3 23 and aPSMP-UTT4 for the non-AP STA4 24. Meanwhile, the PSMP-DTT and/or thePSMP-UTT may be determined as a start time point and duration of acorresponding DTT and UTT. Therefore, the PSMP recovery frame mayfurther include PSMP-UTT offset information indicating a new time pointat which the PSMP-UTT starts.

Upon receiving the PSMP recovery frame, the non-AP STA2 22 can transmita data frame to the AP 10 during a newly allocated PSMP-UTT2 (stepS322). Subsequently, the non-AP STA3 23 and the non-AP STA4 24 cantransmit data frames to the AP 10 during the PSMP-UTT3 and thePSMP-UTT4, respectively (steps S323 and S324).

The PSMP frame is one type of action frame used in a WLAN system.Information included in a frame body may be as shown in Table 1 below.Details of the PSMP frame format may be found in the section 7.4.10.4 ofIEEE 802.11n-2009 Part 11: Wireless LAN Medium Access Control(MAC) andPhysical Layer(PHY) Specification, Amendment 5: Enhancements for HigherThroughput.

TABLE 1 Order Information 1 Category 2 Action 3 PSMP Parameter Set 4 to(N_STA + 3) PSMP STA information It is repeated according to an N_STAsubfield value (N_STA is a subfield of a PSMP parameter set)

Referring to Table 1 above, a category field of the PSMP frame may beset to a value for indicating an HT. The action field may be set to avalue for indicating that a corresponding frame is the PSMP frame.

A PSMP parameter set field is used to define the number of PSMP STA infofields included in the PSMP frame. That is, it can be used for thepurpose of indicating whether a specific PSMP sequence is subsequentlyfollowed by another PSMP sequence and for the purpose of indicating aduration of the PSMP sequence. A PSMP parameter set may include an N_STAsubfield including the number of non-AP STAs, a more PSMP subfieldindicating whether there the PSMP sequence is subsequently followed byanother PSMP sequence, and a PSMP sequence duration subfield indicatinga PSMP sequence duration.

The PSMP STA info field includes information for a PSM operation basedon the PSMP of FIG. 2 and information indicating a PSMP-DTT, a PSMP-UTT,corresponding non-AP STAs, etc. This field will be described in detailwith reference to FIG. 4 and FIG. 5 below.

FIG. 4 is a diagram showing an example of a PSMP STA info field format.The PSMP STA info field format of FIG. 4 is a format capable ofallocating a PSMP-DTT for one or more non-AP STA groups.

Referring to FIG. 4, a PSMP STA info field 400 includes an STA_INFO typesubfield 410 including information indicating a type of the PSMP STAinfo field 400, a PSMP-DTT start offset subfield 420 includinginformation indicating a start time point of a PSMP-DTT, a PSMP-DTTduration subfield 430 including information indicating a PSMP-DTTduration, and a PSMP group address ID subfield 440 including informationindicating a destination non-AP STA group for which a PSM operation isperformed on the basis of information included in the PSMP STA infofield 400.

The STA_INFO type subfield 410 indicates a specific information typecorresponding to a format of the remaining subfields subsequent to theSTA_INFO type subfield in a structure of the PSMP STA info field 400.The PSMP STA_INFO field of FIG. 4 includes PSMP-related informationsupporting a PSM operation of a non-AP STA group, and thus can be set toindicate the PSMP-related information, or can be set to “1” which is avalue for indicating the PSMP-related information.

The PSMP-DTT start offset subfield 420 indicates a PSMP-DTT start timepoint. The time point can be set to a relative value for an end point ofthe PSMP frame.

The PSMP-DTT duration subfield 430 indicates a PSMP-DTT duration, andcan be set to a relative value for an end point of a last PPDU includingdownlink data for a time point at which the PSMP-DTT start offsetsubfield 420 starts.

The PSMP group address ID subfield 440 can be set to indicate 43-bitleast significant bits (LSBs) of a MAC address having a size of 48 bits.

Although the PSMP STA info field 400 of FIG. 4 includes information on anon-AP STA group, the PSMP STA info field can also be configured toinclude information on individual non-AP STAs. This will be describedbelow with reference to FIG. 5.

FIG. 5 is a diagram showing another example of a PSMP STA info fieldformat. The PSMP STA info field format is a format capable of allocatinga PSMP-DTT and a PSMP-UTT to a specific non-AP STA.

Referring to FIG. 5, a PSMP STA info field 500 includes an STA_INFO typesubfield 510 including information indicating a type of the PSMP STAinfo field 500, a PSMP-DTT start offset subfield 520 includinginformation indicating a start time point of a PSMP-DTT, a PSMP-DTTduration subfield 530 including information indicating a PSMP-DTTduration, an STA-ID subfield 540 including information indicating adestination non-AP STA group for which a PSM operation is performed onthe basis of information included in the PSMP STA info field 500, aPSMP-UTT start offset subfield 550 including information indicating astart time point of the PSMP-UTT, and a PSMP-UTT duration subfield 560including information indicating a PSMP-UTT duration.

The STA_INFO type subfield 510 indicates a specific information typecorresponding to a format of the remaining subfields subsequent to theSTA_INFO type subfield in a structure of the PSMP STA info field 500.The PSMP STA_INFO field of FIG. 5 includes PSMP-related informationsupporting a PSM operation of a specific non-AP STA group, and thus canbe set to indicate the PSMP-related information, or can be set to “2”which is a value for indicating the PSMP-related information.

The PSMP-DTT start offset subfield 520 indicates a PSMP-DTT start timepoint. The time point can be set to a relative value for an end point ofthe PSMP frame.

The PSMP-DTT duration subfield 530 indicates a PSMP-DTT duration, andcan be set to a relative value for an end point of a last PPDU includingdownlink data for a time point at which the PSMP-DTT start offsetsubfield 520 starts.

The STA-ID subfield 540 includes information for identifying an STAwhich will perform a PSM operation according to the PSMP STA info field500. This information may be an association ID (AID) which is allocatedwhen the non-AP STA is associated with the AP.

The PSMP-UTT start offset subfield 550 indicates a start time point ofthe PSMP-UTT. The time point can be set to a relative value for an endpoint of the PSMP frame. A first PSMP-UTT can be configured to startwhen a time corresponding to short interframe space (SIFS) elapses froman time point at which the PSMP-DTT ends

The PSMP-UTT duration subfield 560 indicates a PSMP-UTT duration, andcan be configured to indicate a maximum length of the PSMP-UTT which canbe allocated for a non-AP STA for performing uplink transmission.

Meanwhile, in order to introduce a DL MU-MIMO transmission technique, itis decided in a next-generation WLAN system to support a throughput of 1Gbps in a frequency band less than 6 GHz. As a result, an AP cansimultaneously transmit data to one or more non-AP STAs. In order tosupport a PSM operation based on a PSMP in addition to newly introducedMU-MIMO, the existing PSMP-based PSM operation method can be modified oradded.

In the conventional HT WLAN system, the PSMP-based PSM operation isperformed according to a PSMP frame including a PSMP STA info field. Asdescribed above, the PSMP STA info field has two types, i.e., a type inwhich a PSMP-DTT is allocated to an STA group and a type in which aPSMP-DTT and a PSMP-UTT are allocated to each non-AP STA.

First, the PSMP STA info field for setting the PSMP-DTT and the PSMP-UTTto each STA needs to be modified to cover a non-AP STA group which is adestination of MU-MIMO transmission in the next-generation WLAN systemsupporting MU-MIMO transmission.

FIG. 6 shows an example of a PSM operation according to an embodiment ofthe present invention. It is assumed in FIG. 6 that a data frame can betransmitted to a plurality of non-AP STAs 21, 22, 23, and 24 by using aMIMO transmission scheme.

Referring to FIG. 6, an AP 10 transmits a PSMP frame to the plurality ofnon-AP STAs 21, 22, 23, and 24 in order to allocate a media accessduration to the plurality of non-AP STAs 21, 22, 23, and 24 which areMIMO-paired with the AP (step S610). The PSMP frame includes informationindicating a downlink transmission time in which the AP 10 can transmita data frame to the non-AP STAs 21, 22, 23, and 24 and informationindicating an uplink transmission time in which the AP 10 can transmit adata frame to the non-AP STAs 21, 22, 23, and 24. However, if a MU-MIMOtransmission scheme is supported, the PSMP-DTT can be commonly allocatedto the non-AP STAs 21, 22, 23, and 24. This is because the AP 10 cantransmit a data frame simultaneously to the plurality of non-AP STAs.

If the PSMP-DTT is allocated to the non-AP STAs 21, 22, 23, and 24, theAP 10 can transmit the data frame to the non-AP STAs 21, 22, 23, and 24by using the MU-MIMO transmission scheme. The AP 10 can transmit thedata frame at least one or more times during the PSMP-DTT (steps S621and S622). The data frame transmitted by the AP 10 can include at leastone or more data blocks. Herein, the data block includes data which isintended to be transmitted to a specific non-AP STA. According to theMU-MIMO transmission scheme, the data block to be transmitted to thespecific non-AP STA can be transmitted through at least one or morespatial streams. Referring to steps S621 and S622 in which the dataframe is transmitted, the data frame transmitted in step S622 does notinclude the data block to be transmitted to a non-AP STA3. In this case,it means that there is no spatial stream to be allocated to the non-APSTA3.

When downlink transmission performed by the AP 10 ends during thePSMP-DTT, each non-AP STA transmits the data frame to the AP 10 duringthe allocated PSMP-UTT (step S630).

Meanwhile, in case of the WLAN system supporting the uplink MU-MIMOtransmission scheme, the PSMP-UTT can be allocated simultaneously to thenon-AP STAs, and similarly to the method of data frame transmissionperformed by the AP 10, a plurality of AP STAs can simultaneouslytransmit the data frame to the AP 10.

FIG. 7 is a diagram showing an example of a PSMP STA info field formataccording to an embodiment of the present invention.

Referring to FIG. 7, a PSMP STA info field 700 includes an STA_INFO typesubfield 710, a PSMP-DTT start offset subfield 720, a PSMP-DTT durationsubfield 730, a destination STA group subfield 740, a PSMP-UTT startoffset subfield 750, and a PSMP-UTT duration subfield 760.

The STA_INFO type subfield 710 indicates a specific information typecorresponding to a format of the remaining subfields subsequent to theSTA_INFO type subfield in a structure of the PSMP STA info field 700.The STA_INFO type subfield 710 of FIG. 7 is a newly introduced format tobe applied to a next-generation WLAN system. Thus, a subfield valuethereof can be set to a value capable of identifying that the PSMP STAinfo field 700 is a new format. For example, the subfield value can beset to indicate ‘0’.

The PSMP-DTT start offset subfield 720 indicates a PSMP-DTT start timepoint. The time point can be set to a relative value for an end point atwhich transmission of the PSMP frame ends.

The PSMP-DTT duration subfield 730 indicates a PSMP-DTT duration, andcan be set to a relative value for a time point indicated by thePSMP-DTT start offset subfield 720 of a time point at which transmissionof a last PPDU including downlink data ends.

The destination STA group subfield 740 receives the PSMP STA info field700, and includes information indicating a plurality of non-AP STAswhich will perform a PSM operation based on information included in thePSMP STA info field 700. The conventional PSMP STA info field 500 ofFIG. 5 includes the STA ID subfield. This is because downlinktransmission is supported by the AP and the non-AP STA in a one-to-onemanner in a WLAN system not supporting MU-MIMO transmission. On theother hand, since the AP can perform downlink transmissionsimultaneously to a plurality of MIMO-paired non-AP STAs in the MU-MIMOWLAN system, PSMP-DTT and PSMP UTT-related information can be reportedto the plurality of non-AP STAs through the PSMP frame. For this, thePSMP STA info field 700 may include information indicating the pluralityof non-AP STAs which are MIMO-paired with the AP. When the AP transmitsa data frame to the plurality of non-AP STA by using MU-MIMO, theinformation indicating the plurality of MIMO-paired non-AP STAs can beconfigured in a format of a group ID which is information indicating atransmission destination non-AP STA group. The group ID can indicate atransmission destination non-AP STA of a PPDU transmitted by the AP byusing the MIMO transmission scheme, and more particularly, can indicatewhether the MIMO transmission scheme is single user (SU)-MIMO orMU-MIMO. In case of the MU-MIMO, it can be configured to indicate theplurality of non-AP STAs.

Although it is assumed in FIG. 7 that the number of non-AP STAs 21, 22,23, and 24 which are MIMO-paired with the AP 10 is 4 and a group ID canbe set to indicate the four non-AP STAs 21, 22, 23, and 24, the numberof paired non-AP STAs may not be limited thereto. Therefore, a format ofa bit sequence constituting the group ID may vary.

Data which is intended to be transmitted by the AP 10 to each of theplurality of MIMO-paired non-AP STAs 21, 22, 23, and 24 may vary.However, since a data field including data is generated to have the samelength for each non-AP STA through a padding process when generating adata frame to be transmitted by the AP 10, one PSMP-DTT subfieldincluding information indicating a PSMP-DTT may be included in the PSMPSTA info field 700. Therefore, upon receiving the PSMP frame, theplurality of non-AP STAs 21, 22, 23, and 24 can determine whether thePSMP STA info field 700 received through information indicated by thedestination STA group subfield 740 is information for these non-AP STAs.

The PSMP-UTT start offset subfield 750 includes information indicating astart time point of a PSMP-UTT. In this case, the start time point ofthe PSMP-UTT implies a start time point of a PSMP-UTT at which acorresponding period first starts among a plurality of PSMP-UTTsallocated to each of the plurality of AP STAs 21, 22, 23, and 24 whichare MIMO-paired with the AP 10.

The plurality of PSMP-UTT duration subfields 760 include informationindicating a duration of each PSMP-UTT. Respective non-AP STAs which aredestinations of MU-MIMO transmission cannot simultaneously transmit datain a WLAN system not supporting uplink MU-MIMO. Therefore, when aPSMP-UTT is allocated to each non-AP STA when transmitting a PSMP frame,the AP needs to allocate the PSMP-UTT individually to each non-AP STA sothat uplink transmission can be performed during different periods. Forthis, a PSMP-UTT duration subfield for each non-AP STA can be includedindividually in the PSMP STA info field 700. The destination STA groupsubfield 740 includes information indicating the four MIMO-paired non-APSTAs, and thus the PSMP STA info field may include a non-AP STA1subfield 761, a non-AP STA2 subfield 762, a non-AP STA3 subfield 763,and a non-AP STA4 subfield 764. As such, the number of PSMP-UTTsubfields may vary depending on the number of paired non-AP STAs, whichcan be known by using information included in the destination STA groupsubfield. In addition, PSMP-UTT durations allocated to the respectivenon-AP STAs may be different from each other, and the non-AP STAs cantransmit the same-length data during the allocated PSMP-UTT.

Second, the PSMP STA info field which allocates the PSMP-DTT for thenon-AP STA group can be modified to be able to support MU-MIMOtransmission. The PSMP STA info field 400 of FIG. 4 includes the PSMPgroup address ID subfield 440. The PSMP group address ID that can beincluded in the PSMP group address ID subfield may have the samestructure as that of FIG. 8.

FIG. 8 shows a PSMP group address ID applicable to an embodiment of thepresent invention.

Referring to FIG. 8, a PSMP group address ID 800 consists of some bitsof a bit-stream constituting a MAC address 80, and can be configuredwith LSB 43 bits. Among the LSB 43 bits, MSB one bit can be used as anindividual/group indication subfield 810 for reporting whether a bitsequence consisting of some parts of a MAC address relates to individualinformation indicating a specific STA or relates to group informationindicating a non-AP STA group. According to a value of theindividual/group indication subfield 810, a subsequent bit-stream can beinterpreted differently.

For example, when the individual/group indication subfield 810 indicates‘1’ in the next-generation WLAN system, it may imply that the PSMP groupaddress ID 800 is included in the PSMP STA info field. If the APtransmits data to a plurality of non-AP STAs in a multicast or broadcastmanner, at least one or more non-AP STAs which receive the PSMP frametransmitted by the AP may confirm a value indicated by theindividual/group indication subfield 810 included in the PSMP STA infofield and may be able to know that the PSMP STA info field includes theaforementioned information of FIG. 4, thereby being able to operate aPSM.

Unlike this, when the individual/group indication subfield 810 indicates‘0’, it may indicate that another piece of identification information isincluded in the PSMP STA information field instead of the PSMP groupaddress ID. For example, it can be known that information indicating agroup of a MU-MIMO transmission destination non-AP STA group isincluded. The information can be implemented in a format of a group ID720. This can be interpreted as the same PSMP STA info field format ofFIG. 9.

FIG. 9 is a diagram showing another example of a PSMP STA info fieldaccording to another embodiment of the present invention.

Referring to FIG. 9, a PSMP STA info field 900 includes an STA info typesubfield 910, a PSMP-DTT start offset subfield 920, an PSMP-DTT durationsubfield 930, an individual/group indication subfield 940, and adestination STA group subfield 950.

The STA info type subfield 910 indicates a specific information typecorresponding to a format of the remaining subfields subsequent to theSTA_INFO type subfield in a structure of the PSMP STA info field 900.Since the PSMP STA info field 900 of FIG. 9 includes information forallocating a PSMP-DTT to a plurality of MIMO-paired non-AP STAs, the STAinfo type subfield 910 can be set to “1” to indicate that it has thesame structure as that of FIG. 4.

The PSMP-DTT start offset subfield 920 indicates a start time point of aPSMP-DTT. The time point can be set to a relative value for an end pointat which transmission of the PSMP frame ends.

The PSMP-DTT duration subfield 930 indicates a PSMP-DTT duration, andcan be set to a relative value for a time point indicated by thePSMP-DTT start offset subfield 920 of a time point at which transmissionof a last PPDU including downlink data ends.

The individual/group indication subfield 940 may indicate thatinformation included in a subsequent field which is set to ‘0’ isinformation corresponding to a destination STA group subfield.

The destination STA group subfield 950 receives the PSMP frame, andincludes information indicating a plurality of non-AP STAs which willperform a PSM operation based on information included in the PSMP frame.The information can be set to a group ID indicating a non-AP STA groupfor receiving a data frame from the AP 10 through a MU-MIMO transmissionscheme.

The PSMP STA info field 900 of FIG. 9 is set to the same STA informationtype as the PSMP STA info field 400 of FIG. 4, but due to a valueindicated by information included in the individual/group indicationsubfield 940, a subsequent subfield can be interpreted as a destinationSTA group subfield including information indicating a transmissiondestination non-AP STA group for receiving a data frame by using aMU-MIMO transmission scheme. However, since a bit length of the PSMPgroup address ID may be different from a bit length of the destinationSTA group subfield, a bit sequence subsequent to the destination STAgroup subfield may be reserved.

Meanwhile, in uplink transmission, the greater the number of MU-MIMOpaired non-AP STAs, the shorter the PSMP-UTT allocated to each non-APSTA. Therefore, it may be unable to transmit a sufficient amount of datato the AP. In addition, if the PSMP-UTT is too short, a mechanismdepending on a PSMP recovery frame is not normally activated, which mayresult in a high possibility of waste of radio resources. Therefore, thepresent invention proposes a separate PSMP STA info field for allocatingthe PSMP-UTT.

FIG. 10 is a diagram showing another example of a PSMP STA info fieldaccording to another embodiment of the present invention.

Referring to FIG. 10, a PSMP STA info field 1000 includes an STA infotype subfield 1010, a PSMP-UTT start offset subfield 1020, at least oneor more PSMP-UTT duration subfields 1030, and a destination STA groupsubfield 1040.

The STA info type subfield 1010 indicates a specific information typecorresponding to a format of the remaining subfields subsequent to theSTA_INFO type subfield in a structure of the PSMP STA info field 1000.Since the PSMP STA info field 1000 of FIG. 10 has a new format includinginformation for allocating a PSMP-UTT to a plurality of MIMO-pairednon-AP STAs, it may have a value ‘3’.

The PSMP-UTT start offset subfield 1020 includes information indicatinga start time point of a PSMP-UTT. In this case, the start time point ofthe PSMP-UTT implies a start time point of a PSMP-UTT at which acorresponding period first starts among a plurality of PSMP-UTTsallocated to each of the plurality of AP STAs which are MIMO-paired withthe AP.

The plurality of PSMP-UTT duration subfields 1030 include informationindicating a duration of each PSMP-UTT. A PSMP-UTT duration subfield foreach of MIMO-paired non-AP STAs can be included individually in the PSMPSTA info field 1000. The number of PSMP STA duration subfields may beequal to the number of non-AP STAs indicated by the group ID included inthe destination STA group subfield 1040.

FIG. 11 is a block diagram showing a wireless apparatus for implementingan embodiment of the present invention.

Referring to FIG. 11, wireless apparatuses 1110 and 1120 respectivelyinclude processors 1111 and 1121, memories 1112 and 1122, andtransceivers 1131 and 1132. The transceivers 1131 and 1132 transmitand/or receive radio signals having a frame format, and implement anIEEE 802.11 PHY layer. The processors 1111 and 1121 are operativelycoupled to the transceivers 1131 and 1132, and implement an IEEE 802.11MAC layer and the PHY layer for performing a method of transmitting aPPDU delivered from the MAC layer. The wireless apparatuses 1110 and1120 may be APs and/or non-AP STAs for implementing the aforementionedPSM operation method or may be apparatuses included therein. It isassumed hereinafter that the wireless apparatus 1110 is an apparatusincluded in the AP, and the wireless apparatus 1120 is an apparatusincluded in the non-AP STA.

The processor 1111 can be configured to implement operations performedby the AP according to the embodiments of the present inventiondescribed with reference to FIG. 6 to FIG. 10. The processor 1111 can beconfigured to generate a PSMP frame including PSM information, totransmit the frame to at least one or more MIMO-paired non-AP STAs in aformat of a radio signal, and to transmit a data frame to the non-AP STAby using a MU-MIMO transmission scheme according to downlinktransmission period information included in the PSM information. Inaddition, the processor 1111 can be configured to receive a data framefrom at least one or more non-AP STAs according to uplink transmissionperiod information included in the PSM information.

The processor 1112 can be configured to implement operations performedby the non-AP STA according to the embodiments of the present inventiondescribed with reference to FIG. 6 to FIG. 10. The processor 1112 can beconfigured to receive a PSMP frame including PSM information from the APand to transition between an awake mode and a sleep mode on the basis ofthe PSM information. While operating in the awake mode, the processor1112 can be configured to receive a data frame from the AP in a downlinktransmission period and to transmit the data frame to the AP in anuplink transmission period.

The processors 1111 and 1121 and/or the transceivers 1131 and 1132 mayinclude an application-specific integrated circuit (ASIC), a separatechipset, a logic circuit, and/or a data processing unit. The memories1112 and 1122 may include a read-only memory (ROM), a random accessmemory (RAM), a flash memory, a memory card, a storage medium, and/orother equivalent storage devices. When the embodiment of the presentinvention is implemented in software, the aforementioned methods can beimplemented with a module (i.e., process, function, etc.) for performingthe aforementioned functions. The module may be stored in memories 1112and 1122 may be performed by the processors 1111 and 1121. The memories1112 and 1122 may be located inside or outside the processors 1111 and1121, and may be coupled to the processors 1111 and 1121 by usingvarious well-known means.

The aforementioned embodiments include various exemplary aspects.Although all possible combinations for representing the various aspectscannot be described, it will be understood by those skilled in the artthat other combinations are also possible. Therefore, all replacements,modifications and changes should fall within the spirit and scope of theclaims of the present invention.

The invention claimed is:
 1. A method for a wireless local area network,the method comprising: receiving, by a station, station allocationinformation for indicating an uplink time and a downlink time from anaccess point (AP), the station allocation information including a typesubfield and an indication subfield, wherein the indication subfieldindicates whether any subfield for a multi-user (MU) group is includedin the station allocation information when the type subfield is set to afirst value, wherein, if the indication subfield is set to the firstvalue, the station allocation information further includes a groupsubfield and a start offset subfield, the group subfield indicating agroup of stations for receiving a data frame from the AP through amulti-user multiple input multiple output (MU-MIMO) transmission scheme,the start offset subfield indicating a starting time to be accessed bythe group of stations; and when the station belongs to the group ofstations, receiving, by the station, the data frame from the AP at thedownlink time indicated by the station allocation information.
 2. Themethod of claim 1, wherein the indication subfield has one bit.
 3. Themethod of claim 1, wherein the group of stations indicated by the groupsubfield is operating in a power save mode.
 4. A device for a wirelesslocal area network, the device comprising: a transceiver configured totransmit and receive a radio signal; and a processor operatively coupledto the transceiver and configured to: instruct the transceiver toreceive station allocation information for indicating an uplink time anda downlink time from an access point (AP), the station allocationinformation including a type subfield and an indication subfield,wherein the indication subfield indicates whether any subfield for amulti-user (MU) group is included in the station allocation informationwhen the type subfield is set to a first value, wherein, if theindication subfield is set to the first value, the station allocationinformation further includes a group subfield and a start offsetsubfield, the group subfield indicating a group of stations forreceiving a data frame from the AP through a multi-user multiple inputmultiple output (MU-MIMO) transmission scheme, the start offset subfieldindicating a starting time to be accessed by the group of stations; andinstruct the transceiver to receive the data frame from the AP at thedownlink time indicated by the station allocation information when thedevice belongs to the group of stations.
 5. The device of claim 4,wherein the indication subfield has one bit.
 6. The device of claim 4,wherein the group of stations indicated by the group subfield isoperating in a power save mode.